PDA

View Full Version : [Question(s)] Replacing stepper motors with servos on ptz



broadband155
04-09-2009, 03:39 PM
Hi All,

My first post here. I have been reading and watching for a while and would really like to get more involved in robotics.

I wonder if anyone can advise me where to start with this project? This is the first part of a much bigger project for me. I have a commercial cctv ptz camera which draws over 600mA at 12V. This is too much for a roving platform. I would really like to replace the steppers with servos to lower the consumption as these seem to be the main culprits.

I am fine with the mechanical part of it but not at all confident with the electronics/software. I want to keep it working with a standard RS485 protocol like Pelco D, so if I could alter the existing control board for now, that might be the easiest way.

Can I somehow intercept the output from the control board that feeds the stepper controller and make it drive a servo, or is this far too difficult?

Is there a board I can use that will work with this but that doesn't need that much programming? I can't imagine so but its worth asking.

Thanks, Neil.

darkback2
04-09-2009, 04:11 PM
Probably not the best person to answer this question, but I would suggest you instead of trying to "get there from here" instead look at replacing both the motors and the control board. Two boards come to mind...one would be the cheap route using an SSC-32 (http://www.trossenrobotics.com/store/p/3191-SSC-32-Servo-Controller.aspx)to control servo motors...This can be controlled fairly easily, and without too much trouble...another would be to use something like a phidget servo controller (http://www.trossenrobotics.com/p/phidget-advanced-usb-servo-controller.aspx)...more expensive, but would probably be easier to program for.

Also...not sure how powerful the servo motors are going to have to be to meet your needs...You might need to do some fabricating in order to get where you want to go...but as you said you already have the mechanical part down.

Good Luck.

DB

Adrenalynn
04-09-2009, 05:57 PM
Hi There, Welcome to the forum!

I think I'm the resident security camera expert, seeing as how I develop software/hardware/systems in the space, sell them in the space, and hold numerous patents in the space... :)

Pelco P/D has nothing to do with RS485. It's just generally carried across that architecture (but doesn't have to be). The RS485 is a physical and signaling layer. Pelco P/D are protocols carried over that architecture, as are numerous other protocols.

There are servos that support communication over RS485/422/232, but none of them know bupkiss about Pelco protocols. And there aren't any servo controllers that I'm aware of that support the Pelco protocol. Pelco assumes a 360 rotation with fine precision (for high zoom) - and that's the land of the stepper, not a servo.

And honestly, if you look at a Pelco Spectra or similar knock-off like I sell, they use teeny near zero torque steppers anyway. You won't find a servo more than a half an ounce or so lighter that can handle the PTZ work, and they won't be 360 degrees - so the advantage is next to nothing.

The housing and backbox is what makes the weight on a security PTZ camera. Heck - even the dome. Strip that puppy down and you'll find that you're in the same weight bracket as any other PT solution and still have the added advantage of zoom (and high pixel density as well as great sensitivity to light). Leave things well enough alone. :) An XBee -> RS232 -> RS485 will grant you wireless control. I've actually designed and built exactly that for wireless telemetry on PTZ cameras [cheaper than the high-end commercial solutions with better reliability and better range]

If you have a "guttable" PTZ, tear it down and you'll see what I mean. That aluminum/brass composite housing is like 90+% of the weight.

broadband155
04-09-2009, 06:16 PM
Hi Guys,

Thank you both for taking the time to reply so quickly. Darkback2, I will take a close look at your suggestions.

Adrenalynn, I appreciate the information. You refer a lot to the weight of the camera but that isn't a concern for me at all. My focus at the moment is lower power consumption. When I queried the manufacturer of the camera, they told me that the steppers are constantly powered in order to hold position. If I use servos, I save that power.

Also, I don't want or need 360 degree rotation. I would say thirty degrees is enough and fifteen degrees of tilt.

I understand some of what Pelco D is but to explain further, I want to use a single channel streaming device like an Axis 241S which outputs RS232/485 and common protocols like PelcoD. If I don't use this or similar device, I really am taking on far too much in one go. Also, I have access to lots of these streaming devices. I think I have a few from different manufacturers sitting around the place.

Darkback, I am not concerned about servo torque as I will proabably gear it down anyway. I don't need the camera to move like most modern ptz units that can do a full rotation in a second or two.

Adrenalynn
04-09-2009, 08:37 PM
Sorry - for some reason, I was thinking you were concerned about weight...

Servos are always receiving power to hold a position. More than a stepper for the same torque, and they "chatter" which will bounce a camera under zoom around, whereas the steppers don't have the pronounced chatter issues.

>> which outputs RS232/485 and common protocols like PelcoD

PelcoD is a protocol. RS232/485 is not. PelcoD is carried OVER RS232/485. It can also be carried over a rusty piece of barbed wire with the right baluns.



Every Sony CCD-based camera draws a minimum of 350mA, even the non-ptz. 800-1200mA @ 12v is typical with 24 IR LED. PTZs don't draw any more than any other camera until you start swinging them around - on a multi-axis pan/tilt, they can pull upwards of several amps. Which is pretty close to what similar servos would draw under similar load... 600mA is a teeny load for any good 1/3" CCD.

[shrug] I think you'll have trouble finding what you're looking for. Were it me, I'd just slap some simple camera on a couple servos/brackets, pick up some commercial servo controller with a gui, and call it a day. You won't save any power drain in reality though. Servos have to remain powered or they go slack.